The invention relates to a method and apparatus for controlling the combustion temperature in a fluidized bed reactor. In fluidized bed reactors, it is necessary to control the temperature within the combustion chamber so that it is within a desired range in order to maximize the efficiency of the combustion process, and minimize pollutants. Typically, the temperature should be controlled so that it is within the range of about 600.degree.-1100.degree. C. For example, in order to provide an optimal sulfur absorption when burning coal, it is desirable to maintain the combustion chamber within a temperature range of 800.degree.-950.degree. C., and preferably within the range of just below to just above 880.degree. C.
There have been a number of proposals for controlling the combustion chamber temperature of a fluidized bed reactor in the prior art. Typical prior art proposals include controlling the amount of excess air; recirculating flue gases back to the reactor; changing the suspension density in the combustion chamber; dividing the bed into separate functional compartments; and positively cooling a part of the bed material with a separate heat exchanger. Each of these prior art proposals have had disadvantages associated therewith, often either decreasing efficiency, or significantly increasing capital expenditures, or both. According to the present invention a method and apparatus are provided which effectively control the combustion chamber temperature, but do so in such a manner that capital investment is not significantly increased, and combustion efficiency is not adversely impacted.
According to one aspect of the present invention, a method of controlling the temperature in the combustion chamber of a fluidized bed reactor which emits off-gases having particulate solids therein, to keep the temperature within a desired temperature range, is provided. The method comprises the continuous practice of the following steps: (a) Separating the solids from the off-gases. (b) Controlling the temperature of the separated solids. (c) Returning the temperature controlled solids from step (b) to the combustion chamber. And, (d) practicing steps (b) and (c) so that the mass and temperature of the returned solids contribute to the heat content within the combustion chamber in such a way as to maintain it within the desired temperature range. The temperature range is about 600.degree.-1100.degree. C., preferably about 800.degree.-950.degree. C. Steps (b) and (c) are preferably practiced by providing a plurality of return conduits, positively cooling solids passing through at least one of the return conduits, and controlling which conduits the solids flow through.
According to another aspect of the present invention, a fluidized bed reactor is provided in which the combustion chamber temperature may be effectively controlled. The fluidized bed reactor comprises the following elements: A combustion chamber having a lower portion into which the fuel is introduced, and an upper portion into which off-gases with entrained solids pass. Means for effecting separation of solids from the off-gases in the upper part of the combustion chamber. A plurality of return conduits extending between the separating means and the lower part of the combustion chamber. At least one of the return conduits having heat transfer surface means associated therewith for positively cooling solids in contact therewith passing through the conduit. And, means for controlling which of the conduits the solids flow through to return to the lower part of the combustion chamber. According to one embodiment of the invention, no positive cooling means is provided in at least one of the conduits, and the controlling means comprises either diverting valve means at the top of the conduits, or a U-shaped passageway between the conduit(s) having cooling means associated therewith and the lower part of the combustion chamber with means for injecting fluidizing air into the U-shaped passageway. According to another embodiment of the invention, all of the conduits have positive cooling means associated therewith. The conduit flow control means may comprise valve means disposed in the top of each of the conduits; or alternatively a passageway with a bend (e.g. U-shaped or L-shaped) disposed between the bottoms of each of the conduits and the lower part of the combustion chamber, and means for introducing fluidizing air into selected ones of the U-shaped passageways.
It is the primary object of the present invention to provide a method and apparatus for the efficient and effective control of the temperature within the combustion chamber of a fluidized bed reactor. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.